The intermediate processing of stock is known as converting. The basic function of the converting industry is to intermediately process flexible materials, such as paper, plastic, cardboard, or light foil for subsequent use in, for example, the packaging industry. A large volume of the stock material to be converted in is roll form. The conversion of these materials includes, for example, printing, slitting, and trimming as the material is conveyed from a supply roll onto a rewind roll. For certain applications, the conversion process may simply comprise rewinding selected lengths of material from a supply roll onto a rewind roll in preparation for further processing. The stock material is often converted using rewinding machines equipped with devices to perform one or more of the selected intermediate processes.
There are two conventional types of rewinders in general use for converting thin, flexible materials such as plastic film. Both types of conventional rewinders have a set of positively driven primary draw rollers which initially draw the thin web of material from a supply roll. The web is then drawn by a rewinding spindle or core from the primary draw rollers over a series of idler rolls which smooth and guide the web onto the spindle or core. One or more converting processes may be performed on the material before it is rewound on the rewind spindle or core. The two conventional types of rewinders differ, however, in the manner by which the material is drawn from the primary draw rollers and transferred to the rewind roll.
The first type of conventional rewinders are known as surface driven rewinders which include one or more positively driven rollers whose surface contacts the outside layer of material being applied to a freely rotatable rewind core. The friction force created by the rotating surface drive rollers on the outer layer of material on the freely rotatable rewind core simultaneously creates a positive pull on the material and rotates the rewind core. The surface drive rollers are driven at a speed slightly greater than the speed of the moving web so as to create a positive pull on the material.
Some materials have a very smooth and slippable surface. To create a friction force of sufficient magnitude for the conventional surface rewind apparatus to draw this type of material from the supply roll and to turn the rewind core, the surface drive rollers must forcibly contact the outer layer of material on the core. Another reason the surface drive rollers must forcibly contact the rewind core is to prevent the accumulation of air in between successive layers of rewound film. The core is mounted in a "floating" axis of rotation above the surface driven rollers. A portion of the contact force results from the weight of the core and the weight of the accumulated material on the core. However, for successful surface-driven rewinding it is still often necessary to provide a mechanical, pneumatic or hydraulic device which sufficiently increases the contact force between the rewind core and the surface drive rollers beyond the force created by the combined weight of the core and rewound material, particularly at the beginning of the rewind process when the weight of the material on the rewind core is not significant.
Another problem with surface driven rewinders is that the aforementioned device for increasing the contact force between the drive rollers and the rewind core must be continuously adjusted during the converting process. As material is continuously applied to the rewind core, the diameter and weight of the roll steadily increases thereby steadily increasing the contact force on the surface drive rollers. Consequently much more air is squeezed out of the outer layers of material than out of the inner layers of material which results in a rewound roll having a soft interior. This phenomenon may cause further problems when the rewound roll is processed or rewound again. Additionally, the rewound roll may have a telescopic or other irregular shape. Thus, an additional mechanical, pneumatic or hydraulic adjustment mechanism must be provided to compensate for the steady increase in the contact force exerted on the surface drive rollers. This adjustment mechanism also adds to the complexity and cost of the surface driven rewinder.
The second type of conventional rewinders are center-driven rewinders which include a positively driven spindle on which the material is wound to produce the rewound roll. The shaft of the spindle is driven by a motor independent of any initial or intermediate draw rolls. The web is pulled by the rotating spindle which continuously winds successive layers of material onto the spindle.
One problem associated with center-driven rewinders is maintaining a constant tension on the web as the weight and diameter of the rewind roll steadily increase. As the diameter of the rewind roll increases, the tangential velocity of the roll also increases. Further, as the weight of the roll increases, an increase in motor torque is required to rotate the spindle. To compensate for these changing conditions, the angular velocity of the spindle must be reduced but the motor torque must be increased.
Few, if any, constant horsepower motors have heretofore been able to provide a constant horsepower output throughout the build-up range of the rewind roll. The motor will, at times, exert excessive tension on the web causing plastic deformation of the web. Conventional center-driven rewinders also exert excessive tension on the web by design to prevent the accumulation of air in between successive layers of material in the rewind roll. While some center-driven rewinders use a freely-rotatable doctor roll contacting the rewinding roll to prevent entrapment of air in between successive layers of material, excessive tension is still created on the web which causes stretching and plastic deformation. Thus, center-driven rewinders are often impractical for use with thin, plastic material.